Recently, following the tendency of electronic devices towards high performance and compact size, a circuit board is required to have a high degree of lamination and high density. As a method for connecting layers within a substrate that can bond integrated circuits (IC) and components at the shortest distance, it is well-known that higher density can be attained through an inner-via-hole connection. In the case of a through hole connection employed for a typical glass epoxy multilayer substrate, the connection is achieved by plating through holes so that it was difficult to connect only the necessary layers. In addition, since the substrate comprises a top layer having an electrode land, this part cannot be used to constitute an electrode land of a surface mounting part. Therefore, due to these restrictions, the mounting density cannot be enhanced easily. In order to solve these problems, methods other than using through-holes are employed, for example, a method of reducing through-holes by disposing holes halfway in a substrate, or a method of improving mounting density by filling conductive paste in through-holes and by further closing up holes disposed in the top layer of a substrate at a plating step. However, due to the complicated producing processes, the above-mentioned methods have cost and quantity production problems.
On the other hand, only the necessary layers can be connected in the inner-via-hole connection, and since there are no through-holes in the top layer of the substrate, the mounting property is also excellent.
When this connecting method is applied to a resin substrate, e.g. to a glass epoxy substrate, a double-sided substrate establishes continuity by filling up through-holes with low-viscosity solvent-type silver paste by means of a printing method, which then is dried and hardened. However, the volume resistivity of this connection is as high as about 10 m .OMEGA..multidot.cm, and the reliability in thermal shock resistance such as in the heat cycle is poor. The viscosity of the paste should be lowered to improve workability in filling the conductive paste in through-holes. In order to achieve this purpose, conventional methods employed were to use large particles for reducing the amount of conductive particles, or to add a solvent with a low boiling point or a reactant type diluent.
However, when an additive amount of conductive particles was reduced, the problem occurred that contact points between fillers decreased and the resistivity of a via-hole-connection became so high that reliability could not be maintained in a test where thermal stress occurs, e.g., in the heat cycle test. With use of the method in which a solvent with a low boiling point or a reactant type diluent was added, these components volatilized and decreased enormously in weight while being hardened through heat-pressurizing. Due to these volatile components, the base material either swelled or lost peel strength to the copper foils of the wiring.
In order to overcome these problems, Published Unexamined Japanese Patent Application (Tokkai-Hei) No. 7-176846 discloses a solventless conductive paste with low viscosity, high conductivity and reliability in conductive connection and resistance to thermal shock.
In the conventional conductive paste comprising linoleic dimer acid glycidyl ester type epoxy resin as a main component, the crosslinking density is low. As a result, though the paste was less viscous and resistant to thermal shock, the water absorption was high and the reliability in conductive connection was insufficient in a high humidity test. Moreover, the adhesion strength of the linoleic dimer acid glycidyl ester type epoxy resin is not so high as an epoxy resin. When a conductive paste comprising mainly this resin was used for filling via-holes, adhesion to wiring copper foils was insufficient.
A well-known method to lower the water absorption of epoxy resin and improve the adhesion strength is blending an epoxy resin that has a low epoxy equivalent, such as bisphenol-type epoxy resin, so that the crosslinking density can be increased. Although such a resin with low epoxy equivalent can decrease water absorption and improve adhesion strength, it has higher viscosity than that of linoleic dimer acid glycidyl ester type epoxy resin or the like. As a result, such a conductive paste cannot be used for filling via-holes.